Trunked RF repeater systems have become a mainstay of modern RF communications systems and are used, for example, by public service organizations (e.g., governmental entities such as counties, fire departments, police departments, etc.). Such RF repeater systems permit a relatively limited number of RF communications channels to be shared by a large number of users--while providing relative privacy to any particular RF communication (conversation). Typical state-of-the-art RF repeater systems are "digitally trunked" and use digital signals conveyed over the RF channels (in conjunction with digital control elements connected in the system) to accomplish "trunking" (time-sharing) of the limited number of RF channels among a large number of users.
Briefly, such digitally trunked RF communications systems include a "control" RF channel and multiple "working" RF channels. The working channels are used to carry actual communications traffic (e.g., analog FM, digitized voice, digital data, etc.). The RF control channel is used to carry digital control signals between the repeater sites and user RF transceivers (radio units) in the field. When a user's transceiver is not actively engaged in a conversation, it monitors the control channel for "outbound" digital control messages directed to it. User depression of a push-to-talk (PTT) switch results in a digital channel request message requesting a working channel (and specifying one or a group of callees) to be transmitted "inbound" over the RF control channel to the repeater site. The repeater site (and associated trunking system) receives and processes the channel request message.
Assuming a working channel is available, the repeater site generates and transmits a responsive "outbound" channel assignment digital message over the RF control channel. This message temporarily assigns the available working channel for use by the requesting transceiver other callee transceivers specified by the channel request message. The channel assignment message automatically directs the requesting (calling) transceiver and callee transceivers to the available RF working channel for a communications exchange.
When the communication terminates, the transceivers "release" the temporarily assigned working channel and return to monitoring the RF control channel. The working channel is thus available for reassignment to the same or different user transceivers via further messages conveyed over the RF control channel. An exemplary "single site" trunked RF repeater system is disclosed in the commonly-assigned U.S. Pat. Nos. 4,905,302 and 4,903,321.
Single site trunked RF repeater systems may have an effective coverage area of tens of square miles. It is possible to provide one or more satellite receiving stations (and a single high power transmitting site) if a somewhat larger coverage area is desired. However, some governmental entities and other public service trunking system users may require an RF communications coverage area of hundreds of square miles. In order to provide such very large coverage areas it is necessary to provide multiple RF repeater sites and to automatically coordinate all sites so that a radio transceiver located anywhere in the system coverage area may efficiently communicate in a trunked manner with other radio transceivers located anywhere in the system coverage area.
FIG. 1 is a schematic diagram of a simplified exemplary multiple-site trunked radio repeater system having three radio repeater (transmitting/receiving) sites S1, S2, and S3 providing communications to geographic areas A1, A2, and A3, respectively. Mobile or portable transceivers within area A1 transmit signals to and receive signals from site S1; transceivers within area A2 transmit signals to and receive signals transmitted by site S2; and transceivers within area A3 transmit signals to and receive signals transmitted by site S3. Each repeater site S1, S2, S3 includes a set of repeating transceivers operating on a control channel and plural RF working channels. Each site may typically have a central site controller (e.g., a digital computer) that acts as a central point for communications in the site, and is capable of functioning relatively autonomously if all participants of a call are located within its associated coverage area.
To enable communications from one area to another, however, a switching network referred to herein as a "multisite switch", must be provided to establish control and audio signal pathways between repeaters of different sites. Moreover, such pathways must be set up at the beginning of each call and taken down at the end of each call. For example, the site controller (S1) receives a call from a mobile radio in A1 requesting a channel to communicate with a specific callee. A caller requests a channel simply by pressing the push-to-talk (PTT) button on his microphone. This informs the site controller S1 via an "inbound" digital control message transmitted over the RF control channel that a working or audio channel is requested. The site controller assigns a channel to the call and instructs the caller's radio unit to switch from the control channel to the audio channel assigned to the call. This assigned channel is applicable only within the area covered by the site.
In addition, the site controller sends the channel assignment to the multisite switch 200 which assigns an internal audio slot to the call. The switch 200 also sends a channel request to other site controllers having a designated callee within their site area. Audio signals are routed such that audio pathways are created to serve the callee(s) and one or more dispatcher consoles 202 involved in the communication. Upon receiving a channel request, these "secondary" site controllers (in the sense they did not originate the call) assign an RF working channel to the call. Each secondary channel is operative only in the area covered by the secondary site controller. The secondary site controller(s) also sends the channel assignment back up to the multisite switch.
Thus, the caller communicates with a unit or group in another area via the multisite switch. The call is initially transmitted to the primary site controller, routed through an assigned audio slot in the switch, and retransmitted by the secondary sites on various assigned channels in those other areas. When the call ends, the primary site controller deactivates the assigned channel for that site and notifies the multisite switch 200 that the call is terminated. The multisite switch 200 propagates an end of call command ("channel drop") to all other site controllers. This releases all working channels assigned to the call and breaks the associated audio rating pathways.
In addition to providing communications between mobile radio units in different areas, the multisite switch 200 provides communications between land-line telephone subscribers and radio units as well as dispatchers and mobile radio units. Land-line telephone subscribers can communicate with radio units by dialing an access number as well as a radio unit (or group) identification number which is routed to the trunked communications system through a central telephone interconnect switch (CTIS) and the multisite switch 200. One or more dispatch consoles 202 is connected to the multisite switch 200 in the same manner as the site controllers 102. Both land-line subscribers and dispatch console operators can issue a channel call request through the multisite switch 200 to a site controller 102 to call for example a mobile radio unit.
Each dispatch console 202 may participate in calls in its area. Thus, when a call comes through the multisite switch 200 from another area to a mobile radio, the switch informs the dispatch console 202 of the call in addition to notifying the corresponding site controller 102. The dispatch operator can then listen or participate in the call. The multisite switch 200 also handles calls to groups of mobile units and/or dispatch consoles by ensuring that the site controllers for all of the callees in the group assign a channel to the group call.
The multisite switch 200 has a distributed architecture. The logical functions and computational workload of the multisite switch 200 are shared by various distributed microprocessor "nodes". Each node is connected either to a site controller 102, dispatch console 202, public and/or private landline telephone exchanges and other components of the overall radio system. The nodes are referred to herein as interface modules and include, for example, Master Interface Modules (MIMs) for the nodes coupled to site controllers and Console Interface Modules (CIMs) for the nodes coupled to dispatch consoles. Each interface module of the multisite switch is supported by a switch controller card operated by microprocessors. All of the cards have substantially the same hardware and are interchangeable. Each card acts as a gateway interface into the distributed switch network.
One of the significant advantages digital communications systems afford is the capability to digitally encode (e.g. encrypt) voice/data communications. This capability is particularly desirable and necessary for calls over RF channels which can be monitored by any suitably tuned radio receiver. Agencies and departments such as local police departments, in particular, require secure private RF communications. Mobile units and/or groups of mobile units can communicate securely over RF communications links using one or more assigned encryption/decryption "keys" in accordance with an encryption/decryption algorithim, (e.g. the DES) to encrypt the voice/data to be transmitted and decrypt the received information. Only those users having digital encryption/decryption capabilities and the necessary "key" can monitor or participate in the encrypted communication.
The interface modules in the multisite switch do not perform any encryption and decryption, instead, these operations occur at the radio units. Typically, the information is digitally encrypted/decrypted at the radio unit and communicated in that form directly over the audio network to/from the sites through the MIM modules. Unfortunately, some users of the multisite switch, such as dispatch console operators and land-line telephone subscribers, do not have digital encryption and decryption capabilities. As a result, a dispatch operator can not monitor, participate in or originate digitally encrypted calls. Similarly, even though a land line subscriber can access the trunked RF communication system for unencrypted or "clear" calls, that subscriber is unable to receive or originate encrypted calls.
The present invention overcomes these drawbacks by permitting users lacking digital encryption capabilities to nonetheless participate in encrypted communications over the multisite switch. More particularly, one or more digital voice interface modules (DVIM) similar in architecture and operation with other switch interface modules (e.g., CIM's and MIM's) are employed to digitally encode and decode calls for multisite switch "analog" type users lacking that capability.
Each DVIM includes plural digital voice interface units (DVIU) with each DVIU having a digital signal processor that digitally encodes information received from analog communications units over the switch via the DVIM and decodes digitally encoded information on the switch audio network to be received by those analog units. For encryption/decryption, each DVIU stores an assigned cryptographic "key" corresponding to a key employed by a particular radio unit or group of radio units to digitally encrypt/decrypt communications. One or more (including multiple pools of) DVIU's may be dedicated to a particular group or groups of radio units, or a DVIU may be assigned dynamically from a pool of available DVIUs. The DVIM selects (1) the DVIU with the appropriate key to handle a particular encrypted communication and (2) whether that DVIU should encrypt or decrypt the received information.
The present invention therefore permits one or more analog communication units to originate and receive digitally encoded calls without themselves each having digitally encoding/decoding capabilities. In other words, analog communication units do not need to be individually retrofitted with digital encoding/decoding hardware and software to participate in digitally encoded calls over the multisite switch. This is particularly advantageous to dispatch console operators because it allows plural communications, some of which may be digitally encoded, to be summed and monitored on a single speaker. Landline telephone subscribers also benefit from digitally encoded communications (e.g. by enhanced security) with radio units in the field simply by dialing, in addition to a system access number, a single key such as a # key to originate and participate in a digitally encoded call.